1. Field of the Invention
The present invention resides generally in the field of differential wheel mounting for railroad cars.
2. Description of Related Art
Wheelsets commonly in use on the majority of freight railroad cars in the United States have two wheels firmly press-fit onto a common axle over a raised wheel seat. This provides a very high torsional rigidity between the two wheels which forces the two wheels to rotate at the same rotational velocity. The wheel set assembly may be supported by either journal bearings out-board of each wheel, normally at the end of the axle, or the bearings may be located in-board of the wheels. The running surfaces of the wheels are generally profiled such that the wheels are effectively conical in shape, at least when new. However, wheel wear normally quickly changes the straight cone to a hollowed cone in which the effective conicity of the running surface varies with the lateral position of the running surface.
The combination of the effective coning of the wheel's running surface and the rigid axle connection between the two wheels provides the basic mechanism by which a conventional axle with two fixed wheels steers itself. If the wheels are laterally displaced from the center of the track, the wheel that is closer to flange contact has a larger rolling radius than the other wheel. Approximately equal and opposite longitudinal forces are generated on the wheels as the wheel with a larger rolling radius pulls the wheelset forward while the wheel with the small rolling radius pulls the wheelset back. This creates a moment than induces a yaw in the wheelset. This yaw tends to “steer” the wheelset toward the centerline of the track. However, this characteristic of self steering can lead to oscillatory instability otherwise known as hunting, especially at high speeds.
Another problem with conventional railroad wheelsets is encountered on curved sections of track. Operation over sections of curved track ideally requires the wheels to rotate at different speeds because the wheel on the outside rail has to traverse more rail than the wheel on the inside rail. When the wheels are coupled together, the different speeds the wheels are forced to rotate cause slippage between one or both wheels and the rails while traversing curves. This slippage causes wear on both the running surfaces of the wheels and the rails and is a significant cause of maintenance to both wheels and track. Furthermore, the constant slipping that occurs on curved sections of track significantly increases the rolling resistance generated by the conventional wheelset.
As a solution primarily to the problem of slippage while traversing curved sections of track, various wheel and axle constructions have been previously disclosed which permit one or both wheels on a common axle to rotate independently of that axle, i.e., differential wheel rotation. However, these previous arrangements have either been complicated and costly or have lacked sufficient durability to be useful or have otherwise not been accepted in the marketplace.
Nevertheless, the search for a viable solution to these problems common to the railroad industry has continued on over the years. With the nation and world possibly turning more to rail travel for goods and people in the future with ever-growing fuel and energy concerns, this need may be even greater in the years to come. That need is the reason and subject of the present invention.